The invention relates to machine tools or manufacturing machines or robots, by which a unit can be positioned in space through the use of a parallel kinematic assembly having force transfer rods adapted to be driven along travel axes.
The use of rods that serve both as force transfer in moving a platform, and as support for the platform, is typical of the principle of parallel kinematics. A tripod is customarily used, for example, as is described in detail in connection with FIG. 1. In one such tripod, two rods are respectively disposed parallel to each other. The ends of each pair of rods are connected to a respective slider. All sliders are moved in the direction of the x-axis by a respective, appropriately disposed linear motor or a servomotor that operates linearly (rack and pinion). Consequently, a total of three motors can move the three sliders with their respective two parallel rods, by transporting the slides along a single common travel axis. The motors are driven in such a manner that the platform, with the respective assigned tool, moves in the desired three-dimensional workspace without the platform tipping.
However, it has already become evident that angular deviations of the platform from its working position can occur simply because of force vectors. In this situation, different forces must be provided in the movement of the individual rods to produce different elongations. In considering the order of magnitude of the position errors it becomes evident that, without additional inputs to the platform, errors of plus or minus 100 μm are possible.
Up to now, a compensation of these angular deviations was only possible through an adjustment of the length of the individual rods for a selected position in the workspace. Moving the platform out from that position modifies the distribution of forces in the rods and, therewith, also the elongation to be compensated. Because the angular deviation of the platform is different for all working positions, the structure permits establish only one optimal position to be definitively by the adjustment of the rods' lengths. On the other hand other positions in the work area are also indirectly influenced thereby, which can work out to be negative or positive. At present, one such optimization process is carried out manually, at great expense, to produce the intended platform position in the optimal manner in a process cycle.
Yet, as well as the static error, it is also computed with dynamic and load errors, however, whereupon it is then entered.